To integrate the wildlings into the North, they need land to settle on. Having them remain nomadic is not going to work – the rest of the North are settled agrarian people and they are not going to put up with wildlings migrating through their land taking whatever they want. It’s a recipe for cattle raids and bushwhacking and endless conflict. 

The problem then becomes, what land do you give them? The Gift is the only “unclaimed” land in the North – giving them any other land means taking it away from the people already on it, and that’s not going to happen without a fight. 

The hill clans make this point very very clearly when they show up in Jon XI of ADWD:

“Lord Snow,” said The Norrey, “where do you mean to put these wildlings o’ yours? Not on my lands, I hope.”

“Aye,” declared Old Flint. “You want them in the Gift, that’s your folly, but see they don’t wander off or I’ll send you back their heads. Winter is nigh, I want no more mouths to feed.”

Settling the wildlings in the Gift is the least bad solution, as Torghen Flint recognizes. It means the wildlings will be on their own land, and so won’t be making incursions into anyone else’s land. It reduces as far as possible the chances that violence will break out between the two groups.